The present invention is generally directed to an implantable cardiac stimulation device which is powered by a depletable power source. The present invention is more particularly directed to such a device which provides reduced shelf current consumption prior to implant.
Implantable cardiac stimulation devices are well known in the art. They may take the form of implantable defibrillators or cardioverters which treat accelerated rhythms of the heart such as fibrillation or implantable pacemakers which maintain the heart rate above a prescribed limit, such as, for example, to treat a bradycardia. Implantable cardiac stimulation devices are also known which incorporate both a pacemaker and a defibrillator.
A pacemaker may be considered as a pacing system. The pacing system is comprised of two major components. One component is a pulse generator which generates the pacing stimulation pulses and includes the electronic circuitry and the power cell or battery. The other component is the lead, or leads, which electrically couple the pacemaker to the heart.
Pacemakers deliver pacing pulses to the heart to cause the stimulated heart chamber to contract when the patient""s own intrinsic rhythm fails. To this end, pacemakers include sensing circuits that sense cardiac activity for the detection of intrinsic cardiac events such as intrinsic atrial events (P waves)and intrinsic ventricular events (R waves). By monitoring such P waves and/or R waves, the pacemaker circuits are able to determine the intrinsic rhythm of the heart and provide stimulation pacing pulses that force atrial and/or ventricular depolarizations at appropriate times in the cardiac cycle when required to help stabilize the electrical rhythm of the heart.
Pacemakers are described as single-chamber or dual-chamber systems. A single-chamber system stimulates and senses the same chamber of the heart (atrium or ventricle). A dual-chamber system stimulates and/or senses in both chambers of the heart (atrium and ventricle). Dual-chamber systems may typically be programmed to operate in either a dual-chamber mode or a single-chamber mode.
Since implantable cardiac stimulation devices are implanted beneath the skin of a patient, they are powered by a depletable power source, such as a battery. When the remaining battery energy capacity falls below a certain limit, referred to as end of life (EOL), the device must be replaced. Further, prior to EOL, as for example 90 days prior to EOL, the battery will reach a remaining energy capacity corresponding to the recommended replacement time (RRT). An RRT indication is generally provided to alert the patient""s physician that EOL is imminent and is timed relative to EOL to afford the physician sufficient time to schedule replacement of the device before EOL is reached.
As may be discerned from the above, the rate of power consumption by the implanted device is an important consideration after the device is implanted. However, of equal importance and often overlooked is the power consumption by the implantable device after manufacture but before implant.
Implantable cardiac devices are essentially all controlled by internal controllers such as microprocessors. They are in turn controlled by downloaded software. After a device is manufactured and downloaded with software, it is fully tested. Once fully tested, it is then placed into a xe2x80x9cshelfxe2x80x9d mode by a non-implantable device, such as a programmer, for operation in a low power consumption shelf mode.
Existent techniques of placing implantable pacemakers into a shelf mode result in shelf mode battery consumption currents on the order of 8 xcexca. With modern day battery technology, this permits a shelf life on the order of one year with a total battery depletion of about 70 mAH. With respect to defibrillators, shelf life on the order of also one year may be expected with a total battery consumption on the order of 104 mAH. This represents the total consumption of 16 mAH for capacitor reformation and 88 mAH for general consumption.
As will be seen hereinafter, the present invention decreases the shelf current consumption to such a small level that it ceases to become a significant factor in determining device longevity. For example, a shelf current consumption of 1 xcexca will deplete the battery of a pacemaker by 70 mAH over 8 years, and 104 mAH for a defibrillator over 4 years. The battery self discharge dominates battery depletion for shelf currents of 1 xcexcA or less.
The present invention provides a processor controlled implantable cardiac stimulation device having reduced shelf current consumption. In accordance with the broader aspects of the present invention, a shelf mode is established within the device by disabling power to all device circuits except a real time clock, a watch dog timer, a telemetry circuit and the processor which is placed into a static state. The telemetry circuit is placed into a standby mode and the duty cycle of the watch dog timer is reduced. The standby mode of the telemetry circuit permits the telemetry circuit to receive commands from an external source for returning the device to a fully powered active mode prior to or during implant.
When the invention is embodied in an implantable defibrillator, the fully powered active mode may be restored periodically for capacitor reformation. Once the capacitors are charged, the device may then be returned to the shelf mode.
The real time clock times capacitor reform periods and keeps track of time related information. When it is time to reform the capacitors, the real time clock initiates the return to the fully powered active mode and/or the capacitor reformation mode.
When the device is in the shelf mode, the watch dog timer detects for microprocessor errors. If a microprocessor error is detected, the device is returned to the fully powered active mode as an alert to the error condition.